1. Field of the Invention
The present invention relates to a roll forming method for welded steel pipes; with said method the common-utilization of rolls can be realized regardless of the wide variation in dimensions of materials to be formed. The present invention, more specially, relates directly to a novel roll forming method for welded steel pipes by setting the pinch-point (which is a specified location for holding the material to be formed with upper and lower rolls) at the boundary area between the target forming zone and formed or unformed zone along the width direction of the material, and by making said target forming zone in contact with the pre-determined caliber of the upper roll which is placed on the bend-inner side but not making the bend-outer side of the target forming zone in contact with the lower roll to perform the bend forming process, so that the bend-formability can be remarkably improved, roll-flows or roll-scratches can be avoided, and accordingly the common-utilization of rolls can be realized at the break-down forming portion for the edge-bend method.
2. Description of the Prior Arts
In the forming of welded steel pipes, a required cross sectional profile will be made by bending the band steel subsequently in the material's width direction using forming rolls. At the breakdown forming portion which indicates the early stage of roll-forming process, the material to be formed (which is the band steel plate) is, in advance, formed into a semi-circular shape. The thus pre-formed material is then processed through the cluster forming and the fin-pass portion, where the material is formed into an open-shape circular cross-sectional profile.
At the aforementioned breakdown forming portion, mainly two types of forming methods are employed such as the roll-flowers as seen in FIG. 1. With the edge-bend method, the material to be formed is divided into several portions along the width direction and is formed portion by portion to approach the final circular shape. (In the followings, the bend-inner side is referred to the position above the steel plate; while the bend-outer side is referred to the position below the steel plate.) On the other hand, in the circular-bend method, the curvature of the total width of the material is increased stepwise.
By either methods just described in the above, one pair of upper and lower contour rolls is normally used in the conventional breakdown forming stand, as seen in FIG. 2A. For example, the pair of rolls is consisted of a lower roll 2 with concave roll surface(caliber) and an upper roll 3 with convex caliber. The target forming zone of the material 1 to be formed(in other words, both sides of bend-inner and bend-outer sides of the material) is pressed in the pre-set roll gap.
By such conventional forming methods, it is required to pre-set the uniform roll-gap which is equivalent to the wall thickness of the band steel plate in order to provide the pre-determined radius of curvature to the material(which is the band steel plate). However, this forming method possesses following practical drawbacks.
Since there is an unavoidable variation in the plate thickness, and moreover the material during the deformation is also subjected to the variation in thickness, even if the uniform roll gaps being equivalent to the plate thickness are pre-set, the roll surface and the band steel plate surface will not be perfectly in contact. They will rather be in contact in a discontinuous manner. Furthermore, the contacting zone as well as the contacting forces changes with time, and it is impossible to predict these variations.
As a result of the aforementioned technical problems associated with the conventional type of bending methods, it is difficult to set the reference diameter of the roll (which is a diameter corresponding to the roll surface portion having an equivalent rotational speed to moving speed of the material in the longitudinal direction). Consequently, it is very difficult to synchronize the rotational speeds of rolls among each stand, and there could be more loss in driving forces as well as their energies. Also, the surface of the final roll-formed product will suffer from the surface scratches and flaws. Furthermore, since the forming load and driving force are asymmetric along the width direction of the material, the band steel plate is apt to twist in the forming process.
Because both lower and upper rolls are used to form a desired roll gap, there is no flexibility to accommodate to the steel pipe forming with various sizes. For example, when either outer diameter or the wall thickness of the product changes, with the edge-bend method, all forming rolls are required to be exchanged. On the other hand, by the circular-bend method, if the outer diameter of the products remains the same value, rolls can be common-employed in forming pipes with various wall thickness (if they are within a certain limit) by adjusting the roll-gaps.
As a result, the circular-bend method is considered to be more practical than the edge-bend method. However, at the time of forming the material with relatively thin wall thickness, the roll-gaps will become to be uneven and constraint on the material is not sufficient, so that the expected formability can not be achieved. In other words, such common-utilizing of rolls might sacrifice the resultant formability.
However, in order to solve the technical problems of the operational performance and productivity associated with the exchanging operation of rolls, technologies have been developed with regard to the common-utilization of rolls. One of the typical example of such advanced methods is the so-called cage-forming mill.
With such type of mills, the common-utilization of rolls has been proposed by arranging a plurality of small-size rolls (cage rolls) instead of the conventional type of cluster rolls. At the breakdown forming portion, like as the case for the conventional type of mills, the common-utilization of rolls is not realized. In order to reduce the number of rolls which have to be exchanged, not only the forming load at the cluster forming portion but also a part of the forming load which is originally carried out at the breakdown forming portion are shifted to the cage-forming portion, so that the number of breakdown forming stands can be reduced.
However, the forming function of the cage rolls is extremely limited. Namely, since the contact area between the cage rolls and the material to be formed is very small, each cross-sectional portion of the material is formed under the non-uniform bending moment. The formability of such free-bending method depends strongly upon the size and material properties of the products. Accordingly, it is extremely difficult to obtain the desired curvature distribution as has been designed.
Particularly, there are many occasions when the over-bending phenomenon exhibits at the central portion of the steel plate, on which the largest bending moment usually acts. Even if the inner roll (which is the convex roll) is employed, it is impossible to make the bending moment uniform.
Because the excess forming work is allotted to the cage forming portion, adverse effects are influencing on the forming function and stability of the entire mill system. Accordingly, there are many problems recognized with these types of roll common-utilizing mill system.
In order to overcome the aforementioned drawbacks associated with the cage-type mill system, a roll-forming method has been proposed (which is, hereafter, referred as to an FF-flexible forming-method), see U.S. Pat. No. 4,770,019. By the FF forming method, the common-utilization of rolls can be realized not only at the cluster forming portion but also at the breakdown forming portion by using a special roll whose caliber is an involute (which changes its radius curvature either continuously or step-wisely) and employing a position-controlling mechanism for transferring and rotating such rolls.
In order to bend the edge portion of the material which is the most difficult portion to form, the roll arrangement as seen in FIG. 2B is normally used for the so-called No. 1 breakdown stand. According to the roll arrangement for the FF forming method, suitable involute curves are provided to the upper roll 3 (convex roll) and the lower roll 2 (concave roll) respectively, corresponding to the limits of inner and outer diameters of all products in concern.
Although the roll gaps formed with these rolls are normally not uniform, the upper and lower roll positions are determined in order to form the favorable roll-gaps at the edge portion of the material 1, corresponding to the size of the product to be formed. By employing this forming method, it is possible to perform edge-bending without any roll exchange, so that the overall mill formability is improved.
However, even with the FF forming method, the circular-bend type forming method is basically used to the stands except the No. 1 breakdown stand. As a result, most of the forming work is still carried out at the cluster forming portion.